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Capillary GC Column Killers – Part 4

31 May 2012

Aqueous Injections

In my final installment of Capillary GC Column Killers, I would like to discuss aqueous (water) injections. There are a lot of misconceptions about injecting water into a GC, and for good reason. Besides the general knowledge that electronics and water should not be combined, many analysts have experienced chromatography issues when injecting water. Because we get so many calls in tech service about injecting water, we even have a FAQ written on the topic (link below).

FAQ:  Can I inject water onto my column? Will it damage the column?

When it comes to aqueous (water) injections, put your mind at ease, as most fully bonded (cross-linked polymer) phase capillary columns will not be damaged by injections of water. However, water injections should be avoided if possible with non-bonded phases, partially bonded phases, or high-polarity phases.

To determine if your column is fully bonded, carefully read the product description provided by the manufacturer. Generally speaking, Restek columns that begin with Rtx-___, Stx-___ or Rxi-___ are all fully bonded. In addition, most MXT columns offered by Restek are also fully bonded. However, Restek columns that begin with Rt-___ are not fully bonded. While the general rule-of-thumb is to avoid injecting water into columns which are not fully bonded, below are a few exceptions:

1.  Porous Polymer PLOT columns. Although these columns are named Rt-Q-Bond, Rt-QS-Bond, Rt-S-Bond, and Rt-U-Bond, water will not damage these columns (or their MXT equivalents). However, never inject water into any Alumina or Molecular Sieve PLOT columns, as they will absorb the water leading to loss of compound separation and retention. (Note: if this happens, don’t be alarmed, this is a reversible process. Columns can be regenerated by heating to their maximum temperature for several hours to remove the water).

2.  If you must use a non-bonded, partially bonded, or high-polarity column for water injections, make sure the column temperature is kept high enough (greater than 80°C) to prevent water from condensing inside the column, or attach a Polar-Deactivated guard column (also known as a retention gap) prior to the analytical column. This will ensure that only water vapor reaches the analytical column.

Polar-Deactivated fused silica guard columns

As mentioned earlier, while water itself will usually not damage most bonded phase columns, non-volatile components in water-based samples, including particulates, salts, sugars, dissolved metal salts and/or suspended metals, can cause degradation of the column or adversely affect its performance. If you notice degradation in chromatography when analyzing water samples, determine if non-volatile impurities are causing the problem (inspect the injection port liner for residue) and take appropriate action to remove any impurities from the sample/extract prior to injection. This can be accomplished through a clean-up/filtration step during sample preparation. If you cannot clean/filter your sample prior to injection, make sure you use deactivated glass wool or a Carbofrit packed injection port liner.

Everything written up to this point assumes you are injecting the water into a heated split/splitless injection port (containing a deactivated glass liner), and doing a split injection. Other injection techniques can be much more challenging when analyzing aqueous samples. On-column injections seem to be the most challenging. As a result, in tech service, we never recommend on-column injections of water samples. While some analysts have no trouble obtaining good chromatography and reproducible results doing on-column water injections, most analysts have multiple issues that seem impossible to correct. This is especially true for on-column injection ports that are only heated by the GC oven (and not heated independently).

While we are on the topic of on-column water injections, if you must do them, we strongly recommend using a Polar-deactivated guard column, even if using fully bonded stationary phases. This guard column (retention gap) will improve wettability when using non-polar stationary phases, and it will help keep non-volatile residue out of all columns, no matter the phase.

I would like to provide just a few final thoughts to those of you who need to analyze water samples via syringe injection.

First, injection port backflash is always a concern. Because water expands to more than 1400 times its liquid volume when vaporized, always inject the smallest volume possible (0.5µL or less) into the largest internal diameter injection port liner available for the make/model of your GC.

Second, I always recommend injecting water using split mode (with the largest split possible) rather than splitless mode (or Direct injection).

Finally, I usually recommend using a Gas-Tight (PTFE-tipped plunger) syringe as most customers report better reproducibility.

 

Hopefully you all have found my series on Capillary GC Column Killers informative. For those of you who may not have read Parts 1-3, the links are below:

Capillary GC Column Killers – Part 1

Capillary GC Column Killers – Part 2

Capillary GC Column Killers – Part 3

Although my list of capillary column killers is not all-inclusive, it does highlight several of the most common column-damaging situations. Hopefully, this information will provide valuable insight into protecting your capillary column from factors which are within the analyst’s control. Thank you.